It is commonplace for a computer, equipped with a keypad and a monitor, to control the operation of a machine or a process. Usually a programmable logic controller (PLC), also containing a computer under control of the first computer, is used to provide the control signals to the apparatus being controlled. In the case of specialized operations or those involving complex calculations, an auxiliary computer is used for those functions. To avoid extra peripheral equipment in the system it is desirable that the auxiliary computer use the same monitor and keypad as the first computer, even though the computers may not necessarily employ the same communication protocols or signal formats. An example of such a system is a balancing machine.
FIG. 1 shows a typical block diagram for a prior art balancing machine control using a video display based man-machine interface (MMI) 10, a programmable logic controller (PLC) 12 and an auxiliary computer or processor such as a computerized balancing instrument (CBI) 14. The MMI 10 consists of a keypad 16 for entering data, a central processing unit (CPU) 18 and a video monitor 20 for displaying data. An RS-232 serial data link 22 couples the keypad 16 to the CPU 18 and a video line 24 connects the CPU 18 video output to the monitor 20. Another RS-232 link 26 transmits data between the CPU 18 and the PLC 12 and yet another RS-232 data link 28 transmits data between the CBI 14 and the PLC 12. The balancing machine 30 is under control of the PLC 12 via input/output coupling 32.
In operation, when a key is pressed on the operator keypad 16, an RS-232 signal is transmitted to the CPU 18. The MMI program in the CPU receives this signal and decides which function the pressed key is requesting. If the key pressed calls for some action of the part of the PLC 12, the CPU will transmit a command to the PLC via the signal link 26 using a communications format which is normally unique to the manufacturer of the PLC 12. The PLC will then execute the requested command. If that command involves the transmission of data or commands to the CBI 14, the PLC will transmit the information over the data link 28 using the same communications format as used between the PLC and the CPU. This scheme requires the programmer of the CBI to write communications software using many different formats in order to support equipment made by a variety of PLC manufacturers.
If the CBI needs to display data on the monitor 20, it must first be transmitted to the PLC 12 via the RS-232 link 28, from there it is transmitted to the CPU 18 which displays it on the monitor. Since the data to be displayed passes through two computer systems, each busy with other tasks, a significant time delay is experienced between the time the data is available at the CBI and the time that it appears of the monitor. Similarly, input commands to the CBI 14 entered through the keypad 16 also have to pass through the two computer systems of the CPU and the PLC to reach the CBI and therefore suffer similar delays.